1. Field of the Invention
The present invention relates generally to code division multiple access (CDMA) communication systems, and more specifically to a CDMA cellular mobile communication system in which frame offsets are used to avoid traffic peak at base station controllers.
2. Description of the Related Art
According to the ELA/TIA Interim Standard IS-95 for code division multiple access communication systems, mobile stations are assigned unique timing offsets, known as frame offsets, by a base station to transmit their frames at such times that there is no concentration of voice traffic between the base Station and the base station controller even when that base station receives calls from a number of mobile stations at the same time.
Since a number of base stations are served by the base station controller and their frame offsets are uncorrelated with each other, their voice packets may still cause a tic peak at the base station controller if their frame offsets are not uniformly distributed over time. This is particularly true of CDMA base station controllers where conversion is necessary between the QCELP coding format of mobile stations and the PCM format of the network.
It is therefore an object of the present invention to provide a CDMA cellular communication system in which traffic peak is avoided.
According to the present invention, there is provided a code division multiple access cellular communication system, wherein mobile stations transmit frames at timing offset from each other according to a frame offset value. The system comprises a base station controller and a plurality of wireless base stations connected by respective communication links to the base station controller In order to avoid traffic peak, the base station controller determines a distribution of frame offset values according to usage counts of the frame offset values simultaneously used by all of the wireless base stations and transmitting a message to a requesting base station, containing the distribution. The requesting base station is responsive to the transmitted message for selecting a frame offset value according to the distribution contained in the message.
In a specific aspect, the base station is arranged to notify the base station controller of the selected frame offset value. The base station controller includes a frame offset memory, and is arranged to: store the selected frame offset value in the frame offset memory; determine a distribution of frame offset values stored in the frame offset memory according to usage counts, each indicating the number of same frame offset values; identify frame offset values of low usage counts as high priority frame offset values and frame offset values of high usage counts as low priority frame offset values; and transmit to the requesting base station an offset distribution message containing the identified frame offset values. The base station includes a frame offset memory and is arranged to store the selected frame offset value in the frame offset memory, determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, select frame offset values of low usage counts as candidate offset values, and determine a frame offset value as the selected frame offset value, using the identified offset values and the candidate offset values.